Cell arrays are a little tricky in Matlab.
Consider this example.

C=num2cell(rand(2,3))
C = 
    [0.4565]    [0.8214]    [0.6154]
    [0.0185]    [0.4447]    [0.7919]

C{1,2}  % this is the contents of this cell (could be a vector or a string)
ans =
    0.8214

C{1:2,2} % this is the contents of these cells - returns multiple
answers!
ans =
    0.8214
ans =
    0.4447

A = C(1:2,2)  % this is a slice of the cell array
ans = 
    [0.8214]
    [0.4447]

A{1}  % A is itself a cell array
ans =
    0.8214



>> C(1:2,2)=0 % can't assign a scalar to a cell array
C(1:2,2)=0
??? Conversion to cell from double is not possible.


>> C(1:2,2)={0;0} % can assign a cell array to a cell array
C(1:2,2)={0;0}
C = 
    [0.4565]    [0]    [0.6154]
    [0.0185]    [0]    [0.7919]


BTW, I use cell arrays for evidence for 2 reasons:

1. [] indicates missing values
2. it can easily represent vector-valued nodes

The following example  makes this clear

C{1,1} = []
C{2,1} = rand(3,1)

C = 
              []    [0]    [0.6154]
    [3x1 double]    [0]    [0.7919]


Hope this helps,
Kevin
